Organic compound materials containing carbon, hydrogen and oxygen used to be considered as insulators and have been extensively used as passive protective layers (e.g. photoresist) for a long time. The discovery of semiconducting organic materials in 1978 marked the birth of a new field “organic electronics” and “organic optoelectronics”. Organic materials, primarily consisting of carbon, oxygen, hydrogen and nitrogen, can be classified into three different groups: monomers (small molecules), polymers and biological molecules according to their molecular weights.
Monomers, the simplest and lightest structures, are individual units consisting of typically 30 or 40 covalently bonded carbon, nitrogen, oxygen and hydrogen atoms. They can sometimes contain metal atoms (e.g. Al, Pt, Cu), which provide some unique optical or electronic properties. Polymers, are usually formed by stringing together monomers in continuous, repeating chains and their molecular weight can be much higher than monomers (several thousands). While forming polymers, these chains can fold and cross in a random manner, which unavoidably cause high density of defects, resulting in much lower mobilities than monomers. Small molecules are organic materials with intermediate molecular weight between monomers and polymers. Usually an organic material with a molecular weight less than 1000 is called small molecule, and is called polymer beyond this value.
Various electronic and optoelectronic devices have been developed using small molecules organic semiconductors or polymers. Some examples are: organic light emitting devices (OLEDs), organic thin film transistors (OTFTs) and organic solar cells. Compared to devices and circuits fabricated using inorganic semiconductors such as silicon (Si) or gallium arsenide (GaAs), the organic devices and circuits have some advantages such as: low fabrication cost, large substrate area and flexibility. Possible applications of the organic devices include: light sources, electronic displays, circuits, photovoltaic energy conversion and optical signal detection.
There are several requirements for the semiconductor materials, either organic or inorganic, for use in electronic and optoelectronic devices: controlled carrier concentration, large carrier mobility, controlled lifetime and thermal stability. Among these, the carrier mobility has strong effects in affecting the performance and efficiency of the devices fabricated. For most of the device applications, it is preferable to have the carrier mobility as large as possible.
In general, the carrier mobilities for many of the organic semiconductors being developed are quite small (in the range from 10−7 to 1 cm2/V-s) due to the structural nature of these materials. For devices constructed using the organic semiconductors with small carrier mobilities, the performance required for applications may not be easily achieved. This is particularly true when parasitic components are present in these devices which can lead to further degradation of the performance. The above-described parasitic components include unwanted resistance components in these devices. One of these is the contact resistance associated with the interface between an organic semiconductor and a counter electrode. When an organic semiconductor is making contact to a counter electrode, there is a contact resistance. This contact resistance is part of the total series resistance (R) of the device which may limit the operation speeds due to charging and discharging of the capacitor (C) associated with this device. Hence, to achieve better performance, it is desirable to have the product of RC to be as small as possible. One practical way to achieve this is through the reduction of value of R since the value of C is determined by the area of the device and may not be reduced without sacrificing the device performance. Another unwanted effect of having a too large a series resistance is the heat loss due to joule heating. If the resistance of contact between the organic semiconductor and the counter electrode is large, significant amount of joule heating will occur at the contact when a current is applied during the operation. This unwanted joule heating may lead to degradation of the device performance.
Some examples of small molecular organic materials include: pentacence, NPB, AlQ3, CuPc, TPD, Irppy, Some examples of polymeric organic semiconductors include: MEH-PPV (Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene]), poly[3-hexylthiophene-2.5diy](P3HT), poly[3-octylthiophene](P3OT), poly[(4-butylphenyl)-diphenyl-amine-4,4-yl] (poly-TPD), and poly[3,3″-didodecyl-2,2′:5′,2″-terthiophene] (PDDTT). The above exemplary organic semiconductors may be adopted for the fabrication of organic semiconductor devices and circuits. To simplify the explanation, the subsequent description will be made using the organic semiconductors MEH-PPV and P3HT. It is noted that MEH-PPV is an organic semiconductor which emits red light when used in an electroluminescence device whereas P3HT is an organic semiconductor which can serve as an active channel layer when incorporated in a thin film transistor.
In an ideal organic semiconductor device, the resistance between the organic semiconductor and the counter electrode should be as small as possible. However, in many organic semiconductor devices the resistance between the organic semiconductors and contact electrodes are as high as 1010 ohms. This high contact resistance often limits the magnitude of current which is allowed to flow through the organic semiconductors and hence the electronic and optoelectronic performance. The large contact resistances between the organic semiconductors and counter electrodes are mainly due to the relatively large bandgap (˜2.0 eV) and high ionization energy (˜5.0 eV) of the organic materials, which results in Schottky barriers with various metals or inorganic semiconductors. To circumvent this problem, high work function metals such as Au are often used as source/drain contacts in organic thin film transistors (OTFTs). Ideally, the Schottky barriers formed between a clean Au electrode and MEH-PPV or P3HT should be smaller than 0.2 eV, which should not introduce a large contact resistance. However, in practical fabrication, contact resistances over a large range of values have been observed possibly due to the surface states at the Au-P3HT interface. These surface states may vary significantly with different surface preparing methods and may not be easily reproduced during the fabrication.
From the above comments, it is clear that the resistance associated with the organic semiconductor and counter electrode should be as small as possible in order to improve the device performance. However, due to the low carrier mobility and high electrical resistance, it is often difficult to reduce the contact resistance between an organic semiconductor and its counter electrode.